Sperm competition, pregnancy initiation and litter size: influence of the amount of copulatory behaviour in Mongolian gerbils, Meriones unguiculatus

Sexual antagonism for testosterone maintains multiple mating behaviour

1. The persistence of multiple mating remains one of the fundamental questions in evolutionary biology. In theory, multiple mating is predicted to improve female fitness cumulatively through direct and/or genetic benefits. However, intra-locus sexual conflicts may potentially constrain or even eliminate these benefits owing to the gender load imposed by sexually antagonistic selection. 2. Here, we tested whether sexually antagonistic selection can maintain the variance in multiple mating behaviour of bank voles (Myodes glareolus) by manipulating the hormone testosterone through artificial selection in the laboratory. Among mammals, testosterone is a sexually dimorphic fitness-related trait under selection and is known to affect mating behaviour. We conducted mating trials in which females derived from family-based selection of testosterone were sequentially paired with four males of different testosterone profiles. 3. We show that artificial selection for high testosterone increased the mating rate of males, but clearly decreased the number of partners that females mated with (and vice versa). Because multiple mating was beneficial for the reproductive success of both sexes, as evidenced by the positive Bateman gradients, the divergent evolutionary interests of testosterone between the sexes can maintain this polygynandrous mating system. 4. Our results highlight how mating rate is concordantly selected in both sexes; however, it is largely influenced by testosterone, which is under sexually antagonistic selection. 5. This study is the first one to emphasise the direct and indirect effects of the endocrine system not only on reproductive physiology and behaviour but also for the evolution of genetic mating strategies in mammals.

Effects of Photoperiod and Food Availability on Growth, Leptin, Sexual Maturation and Maintenance in the Mongolian Gerbils (Meriones unguiculatus)

Reproductive activity of Mongolian gerbils is regulated by photoperiod nevertheless body weight regulation is controlled without ambient photoperiod. Food intake is a major factor affecting rodent reproductive efficiency. Leptin is a hormone secreted by adipose tissue and modulates food intake, energy expenditure and body fat stores. In this study we studied the interaction of photoperiod and food availability on growth, sexual maturation and leptin concentration in the male and female gerbils. Gerbils were gestated and reared in either 14L:10D or 2L:22D. At weaning, gerbils were housed individually and divided into three groups: fed ad libitum, fed 80% of ad libitum or fed 60% of ad libitum. Body weights were recorded at weaning and every week thereafter. After twelve weeks of treatment, animals were sacrificed and testes and uterine weights were determined and blood was collected for leptin measurement. Food restriction reduced body weight and inhibited reproductive development. Absolute paired testis weights were similar in ad lib and 80% of ad lib groups but significantly different compared with the 60% of ad lib group in both photoperiods. Body weights were also directly dependent upon the level of food restriction. Uterine mass was only affected in the 60% of ad lib group in 14L but both food restriction levels significantly affected the uterine weights in 2L. Significant variations were found in leptin profiles. Leptin concentration was highest in ad lib and 80% of ad lib groups and lowest in 60% of ad lib groups. These results suggest that the reproductive activity of Mongolian gerbils is sensitive to food intake and multiple potential environmental cues (e.g., food availability, temperature) can be utilized.

Overt and covert competition in a promiscuous mammal: the importance of weaponry and testes size to male reproductive success

Male contests for access to receptive females are thought to have selected for the larger male body size and conspicuous weaponry frequently observed in mammalian species. However, when females copulate with multiple males within an oestrus, male reproductive success is a function of both pre- and postcopulatory strategies. The relative importance of these overt and covert forms of sexual competition has rarely been assessed in wild populations. The Soay sheep mating system is characterized by male contests for mating opportunities and high female promiscuity. We find that greater horn length, body size and good condition each independently influence a male's ability to monopolize receptive females. For males with large horns at least, this behavioural success translates into greater siring success. Consistent with sperm-competition theory, we also find that larger testes are independently associated with both higher copulation rates and increased siring success. This advantage of larger testes emerges, and strengthens, as the number of oestrous females increases, as dominant males can no longer control access to them all. Our results thus provide direct quantitative evidence that male reproductive success in wild populations of mammals is dependent upon the relative magnitude of both overt contest competition and covert sperm competition.

Level of energy intake affects the estrous cycle in Sundevall's jird (Meriones crassus)

Effects of energy intake on the estrous cycle of the desert gerbillid, Sundevall's jird (Meriones crassus; 80 g; n=22) were studied. Females were offered either maintenance or below maintenance levels of millet seeds and ad lib. Atriplex halimus leaves and stems; drinking water was not available. Vaginal smears were used to determine sexual stage. We hypothesized that the estrous cycle ceases at low levels of energy intake and commences when sufficient energy is available. Females lost body mass linearly with a decrease in metabolizable energy intake. Estrous cycle averaged 4.46 d at maintenance energy intake but increased to an average of 7.81 d at 70% of maintenance energy intake. A cessation of the cycle occurred at an energy intake below 70% of maintenance requirements, which resulted in a body mass loss of more than 1% per day. More variability in the length of the different stages of the cycle was found with lower levels of energy intake. When offered ad lib. millet seeds and A. halimus (n=14), recovery to the normal cycle was attained within 10 d by 43% of the females and within 16 d by the rest of the females. Recovery time was longest in females that previously had the lowest energy intake. We concluded that the estrous cycle of M. crassus is sensitive to energy intake. With restricted energy intake, the estrous cycle and reproductive activities cease but can be restored with provision of adequate energy. This strategy ensures that reproduction occurs when conditions of food availability and body condition of the females are favorable.

Why some male Mongolian gerbils may help at the nest: testosterone, asexuality and alloparenting

In previous studies we have shown that those male Mongolian gerbils, Meriones unguiculatus, that as fetuses resided in intrauterine positions (IUPs) located between two female fetuses (2F males) have lower circulating levels of testosterone, less well-developed genital musculature, and lower reproductive success than males gestated in IUPs between two male fetuses (2M males). We have also found that such 2F males spend more time caring for conspecific young than do 2M males, and that presence of a 2F male, but not of a 2M male, with a lactating female and her litter decreases the cost to a dam of suckling one litter while gestating a second litter conceived in postpartum oestrus. Here we show that some 2F males, those with circulating levels of testosterone similar to those seen in females of their species, show no interest in females in oestrus and fail to impregnate females with which they are paired. Such 'asexual' 2F males spend 30-50% more time caring for nestlings than do sexually active 2F males. We suggest that such asexual, highly parental 2F males are incapable of direct reproduction and are obligate helpers at the nest that can contribute to their own fitness only by assisting to rear collateral kin. Copyright 2000 The Association for the Study of Animal Behaviour.

Importance of gender for the display of social impairment in pairbond disrupted gerbils

Gerbils form stable male/female pairbonds. Disruption of these has been shown to have consequences for subsequent social behaviour. The response in males is typically a decrease in the initiation of social contacts, whilst females typically freeze when approached. The current studies examined the influence on these phenomena of the gender of the stimulus animal. Data revealed that a similar increase in immobile-in-contact was seen in pairbond disrupted females regardless of whether they were interacting with males or females. By contrast, reductions in social behaviour were only seen in males when interacting with other males. Social investigation was increased when interacting with females. These results are interpreted in the context of gerbil social organisation in the wild. That is, whilst females continue to represent a possibility for opportunistic mating, decreased social investigation and offense directed towards other males indicates a reduced capacity for territorial defence. Together, present findings demonstrate that there are clear sex-differences in the behavioural effects of pairbond disruption in gerbils but that it is only in males that the expression of these effects is influenced by the sex of the stimulus animal.

Pair-bond disruption in Mongolian gerbils: effects on subsequent social behaviour

Gerbils are social rodents which form stable male-female pairs. These pair bonds appear to be based on prolonged association rather than an exclusive mating relationship. However, both sexes contribute to territorial defense and pup rearing. Therefore, pair-bond disruption may be expected to have consequences for subsequent behaviour. This hypothesis was examined in a series of laboratory studies. Behavioural consequences of pair-bond disruption were seen in both sexes and the optimum housing parameters were found to be to pair males and females for 5 weeks and to examine their behaviour 1 week after pair-bond disruption. Two further studies using these parameters were conducted to examine the combined influence of pair-bond disruption and the sex of the animal being interacted with. Results across all studies revealed a consistent pattern. Females showed increased immobility in contact, that is, freezing upon another animal's approach, and altered cage-orientated behaviour, regardless of the sex of the animal they were interacting with. In contrast, males showed decreased social investigation and offense and initiated fewer social interactions. However, these effects were only seen in interactions with other males. No changes in social behaviour were evident in interactions between pair-bond-disrupted males and females. The behavioural strategies of pair-bond-disrupted males and females in a social situation therefore differ. From these studies, it may be concluded that pair-bond disruption has consistent and reproducible effects on subsequent social behaviour in gerbils and that there are important sex differences in the behavioural expression of this.

Reproductive profiles of adult Mongolian gerbils gestated as the sole fetus in a uterine horn

We compared reproductive profiles of Mongolian gerbils gestated alone in a uterine horn (Isolate males and Isolate females) with those of gerbils gestated in intrauterine positions between two male fetuses (2M males and 2M females) and two female fetuses (2F males and 2F females). We found that, when adult, the reproductive profiles of gerbils that had been gestated as isolates resembled that of gerbils that had been gestated as 2F fetuses: 1. Isolate females gestated litters containing both the same proportion of males as the litters of 2F females and a significantly smaller proportion of males than litters of 2M females. 2. Isolate males, like 2F males, were less likely to impregnate females than were 2M males, and 3. both 2F males and isolate males exhibited disturbed patterns of copulation and reduced levels of scent-marking relative to 2M males. Our results were entirely consistent with the view that intrauterine exposure to males, but not to females, was responsible for previously described differences in the reproductive profiles of 2M and 2F Mongolian gerbils.

The evolution of eutherian spermatozoa and underlying selective forces: female selection and sperm competition

We have examined sperm morphology and dimensions in Eutherian mammals. In most Eutherians, sperm heads are round or oval and spermatozoa have short tails (average sperm length about 65 microns; range = 33-121 microns). Rodents, however, clearly depart from the typical Eutherian pattern in that they show a broad array of head morphs and an extreme range of sperm dimensions (35-250 microns). In order to trace the evolutionary changes that rodent sperm have undergone, we have used phylogenetic relationships based on biogeographical, morphological, chromosomal and genic data, and we have superimposed onto them the information available on sperm traits. Analyses were carried out for five rodent groups on which enough information was available. The evolutionary trends which emerged from these studies have two main points in common: throughout evolution spermatozoa have become enlarged and morphologically more complex, and this process seems to have taken place independently in different lineages. A general model was developed which outlines the different evolutionary pathways that rodent sperm have undergone. The adaptive significance of the increase in head complexity and the elongation of the sperm tail remains obscure. We have integrated information from evolutionary, physiological and behavioural studies to address this issue. We argue that two main selective forces may have favoured these changes: female selection within the reproductive tract and sperm competition. The female tract represents a formidable barrier for spermatozoa and its provides an environment where numerous interactions take place. The extent of these barriers and the complexity of these poorly understood interactions suggest that females may be exercising a strong selection, which may enable them to favour particular types of spermatozoa or ejaculates from particular males. Throughout their evolution males must have evolved adaptations to overcome these barriers, and the conflicting interests of choosy females. Sperm competition is a potent evolutionary force among mammals, which has influenced not only the evolution of sperm numbers but also changes in sperm dimensions. Thus, sperm competition has favoured the elongation of the sperm tail, which has led to the attainment of faster swimming speed, an important factor when sperm from rival males compete to reach the ova first.

Modes of estrus induction as a factor in studies of the reproductive behavior of rodents

The choice of a mode of estrus for use in studies related to reproductive processes often is made arbitrarily. Females may be in hormone-induced estrus (HIE), cycling estrus (CE), male-induced estrus (MIE), or postpartum estrus (PPE). Although mode of estrus affects various aspects of copulatory behavior in different species in different ways, males mating with PPE females generally require more intromissions to reach ejaculation than with females in HIE or CE; males mating with females in CE generally attain more ejaculations than with PPE females. The duration of receptivity generally is shorter in PPE than in CE. Females in CE and PPE have different stimulus thresholds for pregnancy initiation, with the direction of the difference varying with the species. Among the other phenomena affected by mode of estrus are sperm competition, mate choice, and the Bruce effect. Because mode of estrus can be an important determinant of results, selection of a mode of estrus for any experiment should not be simply a matter of convenience. Further study of effects of mode of estrus is warranted.